Twist beam axles of the type in question, which comprise trailing arms and a torsional profile connecting the trailing arms, are used for the suspension and wheel guidance of the rear wheels of motor vehicles. Twist beam axles have the advantage, in particular, of a simple, weight- and cost-saving design, wherein, in most cases, no additional assemblies other the trailing arms and the torsional profile are required for the wheel guidance, in particular eliminating the need for additional suspension arms. The need for an additional anti-roll stabilizer is also usually eliminated, since the function thereof is also performed by the twist beam axle.
Twist beam axles require, in particular, a fixed or rotationally fixed and loadable connection between the rigid trailing arms and the relatively soft torsional profile, which undergoes considerable twisting in the event of a unilateral spring compression of a wheel or rolling motions of the auto body. In addition, the trailing arms and the torsional profile are produced, in part, of different materials, for example of cast iron (trailing arms) or sheet steel (torsional profile).
The connection between the torsional profile and the trailing arms in twist beam axles of the type in question, which are known from EP 1 314 587 A2, for example, is typically produced by means of a welded joint. If dissimilar materials are used, however, such as cast iron and sheet steel, for example, conventional welding methods cannot be used.
In fact, the pair of materials “cast iron/sheet steel” requires special welding methods (e.g. laser welding or the Magnetarc process) and weld seam post-treatment in order to alleviate material stresses produced by welding. In order to achieve high-quality laser welding nonetheless, conventional additives and a welding process having a slow advancing speed in a protective-gas atmosphere are required. The welding machines are therefore associated with high costs and require much handling time. Moreover, laser welding with a filler material (which is typically fed in wire form) is a process that is difficult to master. In addition, in the case of cast iron with spheroidal graphite, ledeburite forms in the fusion phase. The structural constituent ledeburite constitutes an unacceptable risk for welded joints of safety-relevant parts, and therefore cannot be used for axle components.
The likewise known “Magnetarc process” (welding using an arc that is moved magnetically), is suited, in principle, for welding safety-relevant parts, since all the ledeburite formed in the fusion process is pressed out of the joining zone and into the weld bead when the components are pressed at the end of the welding process. In this case, however, post-treatment of the weld seam by means of brief tempering is required for the reformation of (unacceptable) martensite. Since the arc is preferably guided in a circle, this joining process is also suited only for certain closed contours. Furthermore, industrial applications of the Magnetarc process are limited to iron-containing components having a wall thickness of less than 6 mm.
Another general aspect of welded joints is that the structural and material properties of the weld seam are typically lower than the corresponding properties of the welded structural components. At the same time, the location of the welded joint is usually subjected to the greatest loads (this applies, in particular, for the combination of the very stiff trailing arms with the torsionally weak cross-profile in a twist beam axle). For these reasons, the weld seams on a twist beam axle are particularly critical with respect to the endurance limit, which is why the quality of the weld seams usually must be inspected, at least by means of random sampling, by means of x-ray examination or using destructive methods. Other paired materials, however—such as light metal and steel or aluminum and fiber-composite materials—cannot be welded together at all, and so, in this case, new ways of connecting the trailing arms to the torsional profile must be investigated.